Passwords are typically used to ensure that only authorized parties have access to information stored digitally in electronic devices such as personal computers. Password entropy, which is usually specified in information bits, is a measure of the degree of protection provided by passwords. Conventionally, password entropy increases with the number of characters required of a valid password—the longer the password, the more difficult the password is to guess, all other things being equal. Nevertheless, reality imposes a practical limit upon the length of passwords, typically because users cannot remember long strings of random characters very well.
Personal computers are not, of course, the only personal electronic devices that store important information digitally. Miniature devices such as personal digital assistants (PDAs) and enhanced-function wireless telephones are now readily available with the capability of storing vast quantities of information and the capability of providing entry to networks of servers and databases. As a result, these miniature devices now require high-entropy password protection.
Miniature devices, however, are subject to another constraint—they are greatly limited in their capability to accept textual input from their users. Whereas a personal computer normally has a full keyboard to accept the entry of a multi-character, high-entropy password, a PDA or enhanced-function wireless telephone may have only a touch screen display that accepts entries made with a stylus. Users of these devices often find it inconvenient and awkward to enter a multi-digit password. Thus there is a need for a way of providing convenient, high-entropy password protection for miniature devices which do not have full keyboards, such as PDAs and cellular telephones.